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Rats vs. Rotifers on New Zealand Islands

Let rats loose on an island and they don’t just scamper around eating birds’ eggs, a study reports in a recent Ecology Letters. They cut nutrient levels, make the soil nearly 100 times less acidic, and topple populations of six out of eight kinds of – well, let’s call them creepy crawlies (springtails, rotifers, nematodes, snails, and so on).

It’s called a trophic cascade – the domino effect of the ecological world. And in this case, rats seem to have pushed against the sort of domino that sets several new rows falling.

The rats weren’t scarfing down tiny soil organisms themselves, or leaching nutrients through some ingenious filtration scheme. All they had to do was drive away the seabirds – graceful, wave-trotting shearwaters and the like – that nest here precisely because there are no predators around.

The study team, led by Tadashi Fukami and composed of scientists from New Zealand, Hawai’i, Alaska and Sweden, chose 18 very similar hunks of rock off northern New Zealand. Rats had jumped ship onto nine of these islands between 50 and 100 years ago. The other nine islands had no rats – and 24 times the number of seabird burrows.

Nitrogen isotopes measured in soil and plant leaves revealed the signature of marine nutrients on the rat-free, guano-splattered islands. Guano splattering is a good thing: carbon, nitrogen and phosphorus levels were at least 45% higher on the rat-free islands. When researchers grew plants in the same greenhouse using soil taken from each of the islands, plants growing in rat-free soil grew to 40% heavier.

Rats did seem to encourage plant growth, with bigger trees, more seedlings and more leaf litter found on invaded islands. The authors suggest that on the other islands, the seedlings may have been kept down by “seabird trampling,” a wonderful thing to imagine.

We live in a world where everything we see runs downstream. So it’s easy to understand problems caused by runoff (dead zones, for example). It’s less straightforward, but more fascinating, to learn of the various ecological rivulets that carry nutrients back up onto land.

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3 Responses

it sounds as if, where the rats were, plant growth was pretty lush. what with no birds crashing about all over the place. but that where the rats weren’t, it would have been even lusher (no leaching and stuff like that) if only the birds could have been persuaded not to walk around.

is this really what you’re telling us or have i miss construed the whole thing?

In a way, yes – the plant growth would likely have been more lush. But that’s not really the point of the article. The soil and the plants themselves contained fewer nutrients where rats had killed off seabirds. That meant that whatever plants were growing – or whatever leaves were rotting in the soil – were less nutritious. And that meant that all of those creepy crawlies I mentioned at the top were much less numerous on the rat-infested islands.

It’s a perceptive question and it points out a bias that we all have: a quite reasonable tendency to assume the importance of large things. Plants are familiar and obvious, so we’re interested when we see them changing. Meanwhile, crawling their stems and burrowing around amid their roots, a complex ecosystem of strange, tiny, unpronounceable animals is starving to death.

At some point, if this were to happen around us, we would notice the absence of these animals because organic matter would take much longer to decompose. In fact, the study suggested that higher accumulations of leaf litter on the rat-infested islands were due to the absence of decomposers.

About the Scribbler

Hugh Powell is a little weary of big-ticket items like Pluto, the Mars rover, and small fossilized humans getting all the science news coverage. Keep an eye out here for wisps and scraps you won't find anywhere else. Particularly about the ocean, which is really cool and, honestly speaking, much bigger than you think.